Various types of devices for manually controlling a position indicator on a visual display are known, e.g. keyboards or push button arrays. With the advent of electronic devices using display screens to inform the user and help him interact with the device, it has become important to provide control devices in which a high-lighted portion of the screen (e.g. cursor or pointer) can be manipulated, i.e. moved to a desired location on the screen. A well known device of this type is a mouse.
A mouse generally contains a ball that is rotatably supported in a housing where the moving of the housing by a user results in a rotation of the ball, because the ball is held in frictional contact with a ground surface underlying the housing. The rotation of the ball is detected by sensor elements that measure the rotation in two different directions. The movement of the ball in these two directions is then translated into a movement of the cursor or pointer on the two-dimensional display screen.
An input device that operates according to basically the same principle is the so-called track-ball. A track-ball also consists of a rotatable ball in a housing, where the rotation of the ball is detected, but the difference is that in the case of the track-ball, it is not the housing that is moved, but the ball itself, which is directly manipulated by the user. This has the advantage that no space is required for moving the mouse about. Consequently, such ball-type devices are preferred in devices where available space has to be economically used, i.e. especially in portable devices like laptop computers.
Reference is made to U.S. Pat. No. 4,939,508 which discloses a point and select device which can function either in a mouse or a track-ball mode according to whether the spherical ball thereof protrudes downwardly or upwardly from the casing of the point and select device.
Various techniques for measuring the rotational movement of the ball are known. One is to provide rollers that contact the ball, where the rollers are oriented to pick up different rotational directions of the ball. The rotation of the rollers is then evaluated, e.g. through optical measurement of scanning disks attached to the shafts on which the rollers are held.
It is also possible to provide an optical method of directly measuring the ball rotation in a compact mouse structure, as shown in WO-94/22071. According to this document, the ball is covered with a plurality of concave reflectors and thereby resembles a golf ball. Light sending and receiving means are provided around the ball, where the varying reflections from the ball are used to detect the ball's rotation. The ball is covered with a transparent layer to thereby be able to roll smoothly. A similar scheme is shown in DE-30 45 133 A1.
Furthermore, a system for the precise measurement of the rotated distance is known from DE-39 12 354 A1, wherein at least two permanent magnets are provided in the rotatable ball, where said magnets cross each other at a non-zero angle. At least two magnetic sensors, such as Hall sensors, are provided around the ball, to thereby measure the ball's rotation on the basis of the change in magnetic field.
The above-mentioned systems are wherein the cursor or pointer can be moved to any desired point on a screen to thereby choose an option associated with that point, where the user then has to actuate a button or key to thereby let the electronic device execute the selected option.
Reference is made to FIG. 1, which is a simplified, very diagrammatic, vertical sectional view through a typical known track-ball for a laptop computer. In this Figure, the track-ball 1 is held captive in any suitable manner, such as between socket members 3, such that it can be rotated in any direction by the user moving his finger or hand when in frictional contact with the upper spherical cap portion of the track-ball that is exposed to the user through a circular opening 4 in the upper side of a housing 2.
The rotation of the track-ball 1 about its axis normal to the plane of FIG. 1 is detected by means of rotation detector 5, which produces an output electrical signal dependent on the track-ball rotation that effects a corresponding displacement of the cursor or pointer along one axis on the display screen or the like. The rotation detector may comprise for example a roller that is in frictional contact with the outer spherical surface of the track-ball.
In addition, a further rotation detector (not shown) is arranged at an angular displacement of 90.degree. typically, with respect to rotation detector 5 about the vertical axis in FIG. 1 passing through the centre of the track-ball. In this way, rotation of the track-ball in any direction is converted by one, the other, or both of the rotation detectors into corresponding output signals that control the motion of the cursor or pointer along one or the other of mutually perpendicular axes, i.e. x and y axes, or at an appropriate angle relative to these two axes.
From a consideration of FIG. 1, it will be appreciated that the bottom of the track-ball is in practice positioned a small distance above the base of the housing, this distance depending upon factors such as the housing design, possibly the space required by the rotation detector, etc. Thus, a track-ball of given size (diameter) will determine the overall height of the ball-type device. It will further be appreciated that because the portion of the track-ball that is exposed to the user must necessarily be less than 50% of the track-ball volume in order that it can be held captive in the housing, a given track-ball size determines a minimum height dimension for the housing. However, particularly where the control device is to be used with a laptop computer, where customer demand is for reduced space requirements, in particular lower height dimensions, a reduced height both for the overall height of the control device and the height dimension of its housing can only be achieved by reducing the diameter of the track-ball. Alternatively, some other form of control device altogether for adjusting the position of the cursor or pointer has to be devised.
As the track-ball size is reduced, the surface area exposed to the user becomes smaller and also its curvature becomes larger. Users prefer a track-ball that is at least of a certain minimum diameter, for ease of use and comfort. Therefore, in practice, there is a limit on how small the track-ball can be made so as to be acceptable or pleasing to the user. This in turn restricts the minimum height dimensions of the control device. Similar considerations apply to attempts to reduce the height of a mouse.